Ultra-wideband (UWB) systems is an emerging technology that offers promise to satisfy the growing demand for low cost and high-speed digital wireless home networks. A traditional UWB technology is based on single-band systems (M. Z. Win and R. A. Scholtz, “Impulse Radio: How It Works,” IEEE Commun. Letters, vol. 2, no. 2, pp. 36-38, February 1998; M. Z. Win and R. A. Scholtz, “Ultra-Wide-bandwidth Time-Hopping spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications,” IEEE Trans. On Commun., vol. 48, no. 4, pp. 679-691, April 2000; M. L. Welborn, “System Considerations for Ultra-Wideband Wireless Networks,” IEEE Radio and Wireless Conf., pp. 5-8, August 2001; J. R. Foerster, “The Performance of a Direct-Sequence Spread Ultra Wideband system in the Presence of Multipath, Narrowband Interference, and Multiuser Interference,” IEEE Conf. on Ultra Wideband Systems and Technologies, pp. 87-91, May 2002; Z. Feng and T. Kaiser, “On Channel Capacity of Multi-Antenna UWB Indoor Wireless Systems,” IEEE Int. Symposium on spread Spectrum Techniques and applications, Sydney, Australia, Aug. 30-Sep. 2, 2004) that directly modulate data into a sequence of pulses which occupy the available bandwidth of 7.5 GHz. Recently, innovative multiband UWB schemes were proposed in (J. R. Foerster, et al., “Intel CFP Presentation for a UWB PHY,” IEEE P802.15-03/109r1, Mar. 3, 2003; E. Saberinia and A. H. Tewfik, “Multi-User UWB-OFDM Communications,” IEEE Proc. On Pacific Rim Conf. on Communic., Computers and Signal Processing, vol. 1, pp. 127-130, August 2003; J. R. Foerster, V. Somayazulu, S. Roy, “A Multibanded System Architecture for Ultra-Wideband Communications,” IEEE Conf. on Military Commun., vol. 2, pp. 903-908, Oct. 13-16, 2003; A. Batra, et al., “Multi-Band OFDM “Physical Layer Proposal for IEEE 802.15 Task Group 3a,” IEEE P802.15-03/268r3, March 2004).
Instead of using the entire UWB frequency band to transmit information, multiband techniques divide the spectrum into several sub-bands. Each sub-band occupies a bandwidth of at least 500 MHz in compliance with the Federal Communications Commission (FCC) regulations (Federal Communications Commission Report FCC 98-153 “Revision of Part 15 of the Commission's rules Regarding Ultra-Wideband Transmission Systems, First report and Order,” Feb. 14, 2002).
By interleaving the transmitted symbols across sub-bands, multiband UWB systems can still maintain the average transmit or transmission power as if a large GHz bandwidth is being used. The advantage is that the information can be processed over much smaller bandwidth, thereby reducing overall design complexity, as well as improving spectral flexibility and worldwide compliance.
A proposal for the IEEE 802.15.3a wireless personal area networking (WPAN) standard (IEEE 802.15WPAN High Rate Alternative PHY Task Group 3a (TG3a). Internet: www.ieee802.org/15/pub/TG3a.html) is based on multiband Orthogonal Frequency Division Multiplexing (OFDM), which utilizes a combination of OFDM and time-frequency interleaving (A Batra, et al, “Multi-Band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a,” IEEE P802.15-03/268r3, March 2004).
The OFDM technique is efficient at collecting multipath energy in highly dispersive channels, as is the case for most UWB channels. Time-frequency interleaving allows the OFDM symbols to be transmitted on different sub-bands. By using proper time-frequency codes, multiband UWB systems provide not only frequency diversity, but also multiple access capability (A. Batra, et al., “Design of a Multiband OFDM System for Realistic UWB Channel Environments,” IEEE Trans. On Microwave Theory and Techniques, vol. 52, no. 9, pp. 2123-2138, September 2004).
To this date, most research efforts on multiband UWB systems have been devoted to the physical layer issues (A Batra, et al, “Multi-Band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a,” IEEE P802.15-03/268r3, March 2004; A. Batra, et al., “Design of a Multiband OFDM System for Realistic UWB Channel Environments,” IEEE Trans. On Microwave Theory and Techniques, vol. 52, no. 9, pp. 2123-2138, Sep. 2004; E Saberinia, J. Tang, A. H. Tewfik, and K. K. Parhi, “Design and Implementation of Multi-Band Pulsed-OFDM System for Wireless Personal Area Networks,” IEEE Int. Conf on Commun., vol. 2, pp. 862-866, Jun. 20-24, 2004; Y. Nakache, et al., “Low-Complexity Ultrawideband Transceiver with Compatibility to Multiband-OFDM,” Technical report, A Mitsubishi Electronic Research laboratory. Internet: www.merl.com/reports/docs/TR2004-051.pdf).
Nevertheless, research and development related to medium access techniques and cross layer design is still limited. Some of the key issues that remain largely unexplored are resource allocations such as power control and channel allocation. The multiband technique (A Batra, et al, “Multi-Band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a,” IEEE P802.15-03/268r3, March 2004) divides the sub-bands into groups, each comprising 2-3 sub-bands. A set of predetermined time-frequency codes is used to interleave the data within each band group. Each user's transmit power is equally distributed among his/her assigned sub-bands. This strategy lacks the ability to allocate sub-bands optimally since the available sub-bands are not assigned to each user according to his/her channel condition.
Since many applications enabled by UWB are expected to be in portable devices, low power consumption becomes a fundamental requirement. The low transmit power of UWB emissions not only ensures long life-time for the energy-limited devices but also reduces co-channel interference. There is a desire to design a proper cross layer scheme that allows UWB systems to operate at a low transmit power level, while still achieving desired performance. In addition, UWB systems are expected to support integration of multimedia traffic, such as voice, image, data, and video streams. This requires a cross layer scheme that is capable of allocating the available resources to a variety of users with different service rates in an effective way. An innovative design of multiband cross layer protocols is important to fully exploit the benefits of UWB systems.
Therefore, adaptive optimization of the sub-band assignment according to channels conditions and power control can greatly improve the system performance of multiband UWB.